Oldham-born but of Welsh descent, Harris has played in 15 GB Tests

But the former Wales rugby union international, who returned to rugby league in 2004, has signalled he could play for Wales at the 2008 World Cup.

"I've enjoyed my time with GB and been very proud to play for them but now is the right time to retire," he said.

"I've had to decide what my priority is and that is to play well for Bulls for the next three to four years."

Harris, 29, said he felt "something had to give" to maintain his standards on a weekly basis, "and that is Great Britain".

My main aim is to win things, as many trophies as possible Iestyn Harris

Harris, who was named in the current squad by coach Brian Noble last week, made 15 appearances for Great Britain after his debut against Papua New Guinea in 1996.

He left Leeds Rhinos in 2001 to play for Cardiff Blues in union, winning 25 caps for Wales and playing in the 2003 World Cup.

But the Oldham-born player returned to league in the summer of 2004, upsetting his old employers Leeds by joining Bradford.

"I've been lucky enough to have missed only one game for Bradford and I want to keep that consistency up while getting the quality higher," he added.

We are completely confident that the overall strength of the squad will not be adversely affected GB coach Brian Noble

"I've enjoyed it here more than anywhere and want to retire at Bradford but my main aim first is to win things, as many trophies as possible, and that's the reason why I'm retiring from international rugby."

Great Britain coach Brian Noble expressed his regret at Harris' decision, but is confident he has sufficient quality in the squad to cover the loss.

"I am sorry that Iestyn will not be available for Great Britain in the future," Noble said.

"He has been an important member of our squad over the last two seasons and has made a solid contribution to what we have tried to achieve. We respect his decision to retire.

"However, we have a great number of high quality players available to us and myself and the other coaches are completely confident that the overall strength of the squad will not be adversely affected.

"We would like to wish Iestyn all the best for the remainder of his career".